Creating a temporary stack allows you to skirt the stack invariant

[jkomoros]

You'd have to work to do it.

But what you'd do is create a new stack of the right deck, move a component to it, and then... just not do anything with the stack and have it get garbage collected.

Maybe the fix is that you can't move things to stacks unless they're part of a SubState. ... Maybe that's already true?

Noticed while thinking through #492